Ultimate Poker became the first legal online gaming service in the U.S. today - at least in Nevada.

The move is significant for companies like Zynga, which are taking steps to supplement their online game businesses with revenues from real-money online gambling. Some companies are already raking in money with "fake-money gambling" games, in which players buy virtual money to make bets, which in return pay out virtual money.

While two other states, New Jersey and Delaware, have passed similar legislation, Nevada is the first to implement it. State residents aged 21 and older can register with Ultimate Poker's website to begin playing.

Internet poker was previously deemed illegal within the US, and in 2011 the Department of Justice seized a number of offshore gambling sites geared toward American players. Las Vegas-based Station Casinos, of which Ultimate Poker is a subsidiary, considers online poker to be a "complement" and not a competitor to casino gambling.

"This is, for Nevada, a new day," says Reno gaming analyst Ken Adams. "There's been a huge amount of speculation on what online gaming means. Estimates in New Jersey run from $20 (million) to $30 million to $2 (billion) to $3 billion. The only way to find out is when it starts. Up to now, we've been talking in theories. Now we'll get a peek at reality."